FIGS. 1(a) and 1(b) illustrate a general squirrel-cage type rotor prior to its casting, in which FIG. 1(a) is a front view showing its cross-section with one part thereof being cut away, and FIG. 1(b) is a side elevational view of the rotor. In the Figures, a reference numeral 1 designates a rotor core formed by lamination, a numeral 1a refers to a slot, 1b denotes a bore for inserting the rotational shaft, and 1c represents a circular steel plate.
So far, this rotor has been manufactured by laminating required number of circular steel plates 1c, each having slots 1a and a rotational shaft inserting hole 1b perforated therein, and then forming the laminated body into a rotor conductor by the aluminum die-cast technique, after which the rotational shaft is inserted into the shaft inserting hole to complete the rotor.
FIG. 2 is a cross-sectional view of a casting apparatus for the conventional cage type rotor, in which a reference numeral 2 designates a virtual mandrel; a numeral 3 refers to a collar; and 4 denotes a nut. An iron cores 1 for the rotor are integrally fastened with the nut 4 by means of the virtual mandrel 2 and the collar 3. A reference numeral 5 designates an extruding rod for taking out a shaped article after its formation; a numeral 6 refers to molten aluminum as the metal material for the rotor conductor; a numeral 7 represents a sleeve for injecting the molten aluminum; 8 refers to a plunger for applying a pressure; 9 denotes a fixed metal mold; 10 represents an intermediate metal mold; and 11 refers to movable metal mold.
The die-cast method for the conventional squirrel-cage type rotor is done in the following manner. That is to say, the iron cores 1 for the rotor, which have been put together integrally by means of the virtual mandrel 2, the collar 3 and the nut 4, are inserted into a cylindrical bore of the intermediate metal mold 10, and then the intermediate metal mold 10 and the movable metal mold 11 are pushed to the fixed metal mold 9 to tightly close the entire mold. Thereafter, the molten aluminum 6 injected into the sleeve 7 is compressed by the plunger 8 at a plunger speed of about 1 m/sec. to flow in and through the slots 1a of the rotor core 1 at a flow rate greater than 1.5 m/sec. and is filled in the slots and the end rings at high speed within a time instant shorter than 1 second and rapidly cooled. After this, the metal mold is opened apart between the fixed metal mold 9 and the intermediate metal mold 10, followed by pushing out the rotor core 1 with the extruding rod 5.
FIGS. 3(a) and 3(b) illustrate the thus manufactured conventional squirrel-cage type rotor, in which FIG. 3(a) is a cross-sectional view, and FIG. 3(b) is a side view thereof. In the Figures, a reference numeral 1d designates an end ring, a numeral 1e refers to a slotted conductor, and 6a represents blowholes. By the way, it is to be noted that the rotor conductor is formed with the end rings 1d and the slotted conductor 1e. Further, FIG. 4 is a micrograph representing the metallographic structure of the rotor conductor (aluminum).
As is apparent from FIGS. 3(a) and 4, there are produced shrinkage holes in the interior of the slotted conductor 1e and the end rings 1d of the rotor after it is subjected to the die-cast operations, which comes into connection with decrease in density. While pure aluminum, for instance, has a density of 2.7 g/cm.sup.3, the density of aluminum of this conventional rotor conductor is 2.57 g/cm.sup.3 at best. The lowered density hinders conduction of the secondary current induced in the rotor, which, furthermore, reduces the rotational torque of the rotor.
At present, therefore, the designing of the rotor is done in such a manner that a safety factor is taken in consideration of decrease in the electric conduction caused by the reduction in density (shrinkage holes), but no thought is given for the full display of the property of the material for the rotor conductor.
In order therefore to obtain the desired motor characteristics, various measures are taken such that thickness of the rotor is increased, the winding for the stator at the primary side is made thicker, and so on. As the consequence of these measures, the motor itself becomes large in size, which not only hinders reduction in its size and weight but also necessitates excess amount of materials, thereby pushing up the production cost of the motor. Further, the blowholes created within the body of the slotted conductor causes decrease in the mechanical strength of the rotor, which are liable to bring the motor into possible risk of wire-breakage and damage during the high speed rotation of the motor.
As mentioned above, with the conventional squirrel-cage type rotor, there have been many points of problem such that the shrinkage holes occur in the interior of the slots and the end rings of the rotor conductor, which bring about decrease in density of the rotor conductor, without being able to attain the high density in the rotor conductor, thereby causing hinderance against increase in the rotational torque of the rotor. Moveover, no limit design is made for the rotor to permit it to display its material characteristics to the fullest extent, which hinders reduction in size and weight and also causes the mechanical strength of the rotor to reduce, which might bring about possible risk of the wire-breakage and damage at the time of high speed rotation.